In a rotating induction apparatus, a current of electrical charge generated within a magnetic field experiences a force perpendicular to the flow of charge and to the lines of force of the magnetic field. If a conductor is forced through a magnetic field by an external prime mover, an electrical current is caused to flow; this is the principle of the operation of an electrical generator. When an electrical current flows through a conductor in a magnetic field, a force is applied to the conductor; this is the principle of the operation of an electrical motor.
In an alternating current (AC) rotating induction apparatus, a rotating magnetic field is produced by the stator or stationary portion of the apparatus. This rotating magnetic field interacts with current carried by conductors of the rotor, causing the rotor to turn. It also produces currents in the rotor conductors by transformer action. Thus, the rotor needs no connections to an electrical supply and is simply supported by bearings which allow free rotation.
The rotating magnetic field is produced by coils, that are made of coils of wire, suitably positioned on the stator. Each coil produces a varying magnetic field of fixed orientation when energized with an AC current. By positioning several coils of differing orientations on a single stator and energizing the coils with alternating currents of differing phase, a rotating magnetic field is produced, which is the sum of the time-varying fixed orientation magnetic fields generated by each coil per phase.
For a rotating induction apparatus having greater than three phases, the coils once wound around the stator may become cumbersome and difficult to wind. As the number of phases increases, the coils become increasingly cumbersome and difficult to wind.